1. Field of the Invention
This invention relates to a method and a system for controlling the characteristics of gradient magnetic fields generated by a magnetic resonance imaging apparatus.
2. Description of the Related Art
A magnetic resonance imaging (MRI) apparatus comprises a unit for generating a static magnetic field; units for applying gradient magnetic fields (Gx, Gy, Gz) in X, Y, and Z directions, respectively; a transmitter for applying an RF (radio frequency) magnetic field to a subject to be examined; a detector for detecting an MR signal generated within the subject; a sequence controller for controlling the generation of the magnetic fields and the detection of the MR signal; and a computer system for controlling the sequence controller and processing the detected MR signal to produce an MR image.
With the MRI apparatus as described above, the subject is placed in the static uniform magnetic field generated by the static field generating unit. The transmitter is driven in accordance with the sequence operations of the sequence controller to apply a 90.degree. pulse and a 180.degree. pulse to the subject. The gradient field generating units also apply the gradient fields to the subject in accordance with the sequence operations of the sequence controller. As a result, the MR signal generated within the subject by an MR phenomenon, is detected by the receiver. The computer system performs image reconstruction processing for the detected MR signal, thus providing MR imaging information such as an MR tomogram image.
It should be noted here that the MR imaging information is affected by the rise characteristic, for example, the rise time, of the gradient magnetic field applied to provide position information to the MR signal. On the other hand, it may be considered that the rise characteristic of the gradient magnetic field depends mainly on the output characteristics of a power source used for generating the gradient magnetic field. Thus, the output characteristics of the gradient field generating power source will be discussed hereinafter with reference to FIGS. 1 and 2.
FIG. 1 shows a power source for supplying a coil with a power source voltage to generate a magnetic field. An input signal Vin is applied to amplifier 1 operated by voltages +V and -V, and amplified therein for supplying coil 2. Coil 2 has an impedance Z.sub.1 (an inductance part Z.sub.l1 and a resistance part Z.sub.r1). Shunt resistor 3 is provided for measuring a current flowing through coil 2. I.sub.out represents a current flowing through coil 2.
The rise time T.sub.r of the current I.sub.out flowing through coil 2 is given by EQU T.sub.r =(Z.sub.l1 /Z.sub.r1) (V/(V-I.sub.out .multidot.Z.sub.r1))(1) EQU V=Z.sub.l1 (di/dt)+I.sub.out Z.sub.r1 ( 2)
As can be seen from the above equations, the rise time T.sub.r depends on the impedance Z of coil 2 and the voltage V applied to amplifier 1. As shown in FIG. 2, current I.sub.out is delayed by time T.sub.r with respect to input voltage Vin. Thus, when the gradient magnetic field generating coils have various impedances, the rise times of the gradient magnetic fields are varied, thereby failing to provide accurate position information to the MR signal.
Therefore, a method is desired in which the characteristics of the gradient magnetic fields can be controlled so as to provide accurate position information for the MR signal.